If the source list is a named range that contains blank cells, users may be
able to type any entry, without receiving an error message. To prevent this:

Select the cell that contains a data validation list
Choose Data|Validation
On the Settings tab, remove the check mark from the Ignore blank box.
Click OK
Note: If the source list contains blank cells, and is a range address, e.g.
$A$1:$A$10, it will block invalid entries with Ignore blank on or off.

Second..........................................

If your list is directly entered as a comma de-limited list you can add a
blank entry by adding a comma <space

i.e. NA,dog,cat,fish,<space


Gord Dibben MS Excel MVP

On Fri, 19 Mar 2010 05:18:00 -0700 (PDT), Tonso
wrote:

I have noticed the "Ignore blank" check box in my Data Validation
window (XL2003), but i cannot figure out what it does. Nor can i find
an answer in the Help files. Just what does this option do?

Thanks,

Tonso